I. Field
The disclosure generally relates to communication systems and, amongst other things, to constrained hopping and channel estimation in wireless communication systems.
II. Description of the Related Art
In digital communications, information is translated into digital data referred to as bits. A transmitter modulates an input bit stream into a waveform for transmission over a communication channel and a receiver demodulates the received waveform back into bits, thereby recovering the information. In an ideal communication system, the data received would be identical to the data transmitted. However, in reality, distortions or noise may be introduced during the transmission of data over a communication channel from the transmitter to the receiver. If the distortion is significant, the information may not be recoverable from the data received at the receiver.
Orthogonal Frequency Division Multiplexing (OFDM) is a modulation technique that effectively partitions the overall system bandwidth into a number of (N) orthogonal subcarriers. The subcarriers are also commonly referred to as tones, bins, and frequency channels.
OFDM is widely used in various communication systems. For example, an orthogonal frequency division multiple access (OFDMA) system utilizes OFDM and can support multiple users. The N subcarriers may be used for data and pilot transmission in various manners, depending on the system design. For example, the OFDMA system may partition the N subcarriers into multiple disjoint groups of subcarriers and allocate each subcarrier group to a different user. Multiple users can then be supported simultaneously via their assigned subcarrier groups.
Data is often distorted during transmission. To mitigate the effects of distortion, channel estimation is one technique used to compensate for the distortion introduced in data during its transmission. Channel estimation is sometimes accomplished by use of a broadband pilot signal, where a fraction of the total available tones are reserved for pilot symbols. These pilot symbols are typically spaced equally throughout the band for optimal performance. At the receiver, the channel response can then be estimated by processing the data received in a distorted manner. If a user needs to estimate multiple channels, such as a user in a multiple-input multiple-output communication system (MIMO), the system overhead increases. For example, in a four antenna MIMO transmission, three additional broadband pilot signals need to be transmitted.
A typical MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission and is denoted as an (NT, NR) system. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS spatial channels, where NS≦min {NT, NR}, as described below. NS data streams may be transmitted on the NS spatial channels. The MIMO system can provide increased transmission capacity if the NS spatial channels created by the multiple transmit and receive antennas are used for data transmission.
The transmission capacity of each spatial channel is dependent on the signal-to-noise-and-interference ratio (SINR) achieved by that spatial channel. The SINRs for the Ns spatial channels are dependent on the channel conditions and may further be dependent on the manner in which the data streams are recovered at the receiver. In one conventional MIMO system, a transmitter encodes, modulates, and transmits each data stream in accordance with a rate selected based on a model of a static MIMO channel. Good performance can be achieved if the model is accurate and if the MIMO channel is relatively static (i.e., does not change much over time). In another conventional MIMO system, a receiver estimates the MIMO channel, selects a suitable rate for each spatial channel based on the channel estimates, and sends NS selected rates for the NS spatial channels to the transmitter. The transmitter then processes NS data streams in accordance with the selected rates and transmits these streams on the NS spatial channels. The performance of this system is dependent on the nature of the MIMO channel and the accuracy of the channel estimates.
When a user's symbols are transmitted in a hopping pattern over the full band, channel estimations need to be carrier out over the entire band. This is exacerbated in the case of a MIMO user, where broadband pilot signals are needed for every channel estimated. Further, having users operate over the entire band reduces channel variations and thus can diminish multi-user gains.
Accordingly, operating users across the available frequency band increases overhead. Further, it does not take advantage of scheduling users in favorable channel conditions. Therefore, there is a need for more efficient methods and systems for allocating resources.